This invention relates to containers, and more particularly to marking components of metal cans with machine readable information.
Two piece metal cans include a can body on which a can end is attached by a seam. Commercial two piece beverage cans are formed by a drawing and ironing process that forms the body sidewall integral with the base. Three piece metal cans include a cylindrical body, each end of which has a can end attached by a seam.
Two piece beverage cans are produced in vast quantities for beverages and foods use; three piece beverage cans are produced in vast quantities for food uses. Accordingly, the components of the cans must be produced at high speeds.
Conventional beverage cans and many easy open food cans have pull tabs. Pull tabs are formed from metal sheet in a tab press. Because of the quantities required, conventional tab presses form multiple tabs at once in lanes of two, three, or four tabs.
Typically, pre-lacquered coil of aluminum is fed into a shell press to form the can end shells. A pre-lacquered coil of aluminum is fed into a tab press to form the pull tabs. The shells and pull tabs are combined in a conversion press to form the unseamed can end.
Decoration of can ends, especially pull tabs, is known. For example, U.S. Pat. No. 6,105,806 discloses laser etching or removal of portions of a coating on a pull tab. U.S. Pat. No. 6,498,318 acknowledges difficulties in marking metal cans and discloses ablating metal pull tab stock.
The inventors are aware of a commercial, conventional system for laser etching pull tabs that includes a CO2 laser that often operates at about 100 W. Each lane of a tab press has its own laser such that the tab press can operate at about 700 tabs per minute with a laser having a resolution or dimension of approximately 100 microns. Typically, a dark coloured lacquer is removed by the laser to expose bare aluminum in the form of a simple logo or a few characters. The limitation on the process speed is also a limitation on the amount of decoration.
Laser marking of coatings is employed for flexible and card packaging for various marking applications. Typically, a thermally active pigment is loaded into a transparent or light colored lacquer, and a CO2 laser induces a color change. For example, a laser may be applied to a white lacquer label to display black text. Laser marking systems of this type are available from Sun Chemical, under the trade name Sunlase and employ a 100 micron YAG laser.